Process for recovery of hydrogen and abstraction of sulfur

ABSTRACT

A process of abstracting sulfur from H 2  S and generating hydrogen is disclosed comprising dissolving Pd 2  X 2  (μ-dppm) 2  in a solvent and then introducing H 2  S. The palladium complex abstracts sulfur, forming hydrogen and a (μ-S) complex. The (μ-S) complex is readily oxidizable to a (μ-SO 2 ) adduct which spontaneously loses SO 2  and regenerates the palladium complex.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the interaction of transition metal complexes,more specifically, palladium complexes with H₂ S. More particularly,this invention discloses a process for removing H₂ S from a feedstocksuch as natural gas by means of a palladium metal-metal bonded dimerthat abstracts the sulfur and generates hydrogen.

Field natural gas typically contains undesirable constituents such as H₂S which must be removed because of its corrosive and noxious nature. H₂S is now removed by treatment with aqueous ethanolamine in acountercurrent cycle followed by regeneration in a stripper column.Amines are often used and typical amines used are diisopropylamine ormethyldiethanol amine. B,B'-hydroxyaminoethyl ether known asdiglycolamine is also sometimes used.

The preferred alkanolamines typically have a hydroxyl group to lower thevapor pressure and to provide water solubility. The alkaline amine groupabsorbs acidic contaminant gases.

The alkanolamines, however, suffer several drawbacks--a problematic andcostly one being the formation of irreversible reaction products withsome contaminants such as COS and CS₂. This results in an economic lossfrom loss of alkanolamine if these contaminants are present in the well.Other drawbacks of alkanolamines include relatively high corrosivity,vaporization losses due to their relatively high vapor pressure, and theneed often to keep water content below 5% which necessitates highreboiler temperatures.

A sweetening process for soured natural gas, able to effectively removeH₂ S but without many of the above attendant drawbacks, would be anadvance in the art.

2. Description of Related Art

The palladium dimer complexes Pd₂ Cl₂ (μ-dppm)₂ have been reported by A.L. Balch, L. S. Benner, and M. M. Olmstead, Inorg. Chem., 1979, 18, 2996and C. L. Lee, B. R. James, D. A. Nelson, and R. T. Hallen,Organometallics, 1984, 3, 1360.

The present invention discloses a new and useful process utilizing thesecomplexes.

SUMMARY OF THE INVENTION

This invention discloses a process for the removal of H₂ S from a gasfeedstock and the conversion of the H₂ S to hydrogen and organosulfurcompounds.

The invention is based on the discovery of the reaction: ##STR1## whereX=Cl, Br, or I.

The palladium complex (1a) is [Pd₂ X₂ (μ-dppm)₂ ], where dppm isbis(diphenylphosphino)methane and X is Cl, Br, or I. The complex (1a) isa bridged dppm dimer, [Pd₂ X₂ (μ-dppm)₂ ], but is written forconvenience as Pd₂ X₂ (dppm)₂. All of the above are to be understoodherein and are defined herein as equivalent representations of the samepalladium complex.

This reaction can be carried out in solution under ambient conditions.Therefore, Pd₂ Cl₂ (dppm)₂ can be used to remove H₂ S from sourednatural gases.

It has been found that H₂ S can be removed from natural gases bybubbling the gas through a CH₂ Cl₂ solution of Pd₂ Cl₂ (dppm)₂ at 10⁻¹-10⁻² M. The palladium complex abstracts sulfur with the concomitantrelease of hydrogen. The sulfur-bearing palladium complex then can beoxidized with a mild oxidant to an SO₂ containing product, Pd₂ Cl₂(μ-dppm)₂ (μ-SO₂), that spontaneously releases the SO₂ with regenerationof the palladium complex.

DETAILED DESCRIPTION

When Pd₂ Cl₂ (dppm)₂ is dissolved in a solvent such as dichloromethaneand exposed to H₂ S, Pd₂ Cl₂ (dppm)₂ (μ-S) is formed together with theevolution of hydrogen gas. Essentially, hydrogen is split from the H₂ Smolecule and sulfur is incorporated between the two palladium atoms. Thereaction can be written as

Pd₂ Cl₂ (dppm)₂ +H₂ S→Pd₂ Cl₂ (dppm)₂ (μ-S)+H₂

or in general as: ##STR2## wherein P P is bis(diphenylphosphino)methane,and X is selected from Cl, Br or I.

Quantitative measurements show that 97% of the theoretical amount ofhydrogen is obtained.

The relative reactivities toward H₂ S of the differing halogensubstituents are:

Pd₂ Cl₂ (dppm)₂ >Pd₂ Br₂ (dppm)₂ >Pd₂ I₂ (dppm)₂.

Pd₂ Br₂ (dppm)₂ reacts considerably slower than Pd₂ Cl₂ (dppm)₂, and theiodo substituted complex slower still.

The commercial significance of the Pd₂ Cl₂ (dppm)₂ route to hydrogengeneration becomes apparent upon the recognition that Pd₂ Cl₂ (dppm)₂can be regenerated from Pd₂ Cl₂ (dppm)₂ (μ-S) by oxidizing the sulfur toSO₂. The complex thus becomes valuable for the quantitative recovery ofhydrogen from H₂ S.

A one step hydrogen separation and sulfur abstraction from gas mixtureswith compositions similar to that of oxygen blown coal gas is madepossible by the invention. Quite generally it can be stated that Pd₂ X₂(dppm)₂ (μ-SO₂) can be generated from Pd₂ X₂ (dppm)₂ (μ-S) by oxidationwherein X=Cl or Br or I. Oxidation can be carried out using oxidantssuch as H₂ O₂, pyridinium chlorochromate, pyridinium dichromate, andm-chloroperbenzoic acid. The SO₂ adduct is formed in high yields whenthe oxidation is performed at -20° C. with two equivalents of oxidant.Oxidation of Pd₂ Cl₂ (dppm)₂ (μ-S) with a slight excess ofm-chloroperbenzoic acid at -20° C. followed by addition of hydrazine toelement excess oxidant resulted in the formation of Pd₂ Cl₂ (dppm)₂(μ-SO₂) in 76% isolated yield.

Though the SO₂ adduct tends to spontaneously lose SO₂ thus regeneratingthe Pd₂ X₂ (dppm)₂ catalyst, the rate and extent of regeneration of thePd₂ X₂ (dppm)₂ catalyst can be maximized or enhanced by using meansremoving SO₂ from the SO₂ adduct. The removal means to liberate SO₂ canbe selected from N₂ gas introduction, application of heat, orapplication of reduced pressure.

The violet Pd₂ X₂ (dppm)₂ (μ-SO₂) (X=Cl, Br, I) complexes lose SO₂ whenN₂ is bubbled through the solution or if subjected to heat or vacuum.The rate of SO₂ loss decreases in the order to I>Br>Cl as judged bycolor change in samples of similar concentration.

                  TABLE 1                                                         ______________________________________                                        PHYSICAL PROPERTIES OF SOLVENTS FOR                                           DISSOLUTION OF Pd.sub.2 Cl.sub.2 (dppm).sub.2                                              Solubility                                                                              Vapor Pressure                                                                            Boiling                                    Solvent      Parameter at 25° C. (torr)                                                                   Point (°C.)                         ______________________________________                                        dichloromethane                                                                            9.80      424         39.7                                       dimethylacetamide                                                                          10.80     ˜0    165                                        diphenylether                                                                              10.10     0.10        258                                        1,2,3,4-tetrahydro-                                                                        9.50      0.38        205                                        naphthalene(tetralin)                                                         dibutylphthalate                                                                           9.85      ˜0    340                                        1,1,2-trichloroethane                                                                      9.88      22.49       113.8                                      1,2,3-trichloropropane                                                                     10.09     3.33        156                                        ______________________________________                                    

Pd₂ Cl₂ (dppm)₂ has a solubility of 0.15M in trichloroethane and 0.027Min trichloropropane.

Solvent choice appears to have an effect on the lifetime of the complexin solution.

Pd₂ Cl₂ (dppm)₂ is soluble in dichloromethane to form a 0.05M solution.The boiling point (39.7° C.) and vapor pressure (424 torr at 25° C.) ofdichloromethane however do not make it the easiest solvent to work with.

Solvents more suitable for dissolution of Pd₂ Cl₂ (dppm)₂ can bedetermined by the concept of solubility parameters as illustrated byHildebrand, J., and Scott, R. L. The Solubility of Nonelectrolytes, 3rdEd., Reinhold Publishing Corp., New York, 1950, and Hoy, K. L. J. PaintTechnol., 42, 1, 1970.

The solubility parameter (δ) is related to the internal pressure orcohesive energy density

δ=(ΔE/V)

where

ΔE=energy of vaporization and

V=molar volume.

The solubility parameter of Pd₂ Cl₂ (dppm)₂ is estimated asapproximately 10 based on its solubility in dichloromethane.

Possible process solvents can be tetralin [though its UV/VIS spectrumoverlaps somewhat with that of Pd₂ Cl₂ (dppm)₂ ], dibutylphthalate, ordiphenylether. Table 1 lists several possible solvents. Based on costs,1,1,2-trichloroethane was preferred for Pd₂ X₂ (dppm)₂, where X=Cl orBr.

The projected economics for hydrogen separation by the process of theinvention are dependent on the lifetime of the palladium catalyst insolution. Complex lifetime appears influenced by the solvent selected.Laboratory experience with trichloroethane as solvent seemed to suggestthat the complex longevity in trichloroethane might be less than optimumin some commercial operations. More favorable complex lifetimes werefound with dichloromethane solvent.

The principles, preferred embodiments, and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited solely to the specific structure and formulasdisclosed, since these are to be regarded as illustrative rather thanrestrictive. Variations and changes can be made by those skilled in theart without departing from the spirit and scope of the invention.

We claim:
 1. A process for abstracting sulfur and forming hydrogencomprising:dissolving in a solvent a palladium complex of the formula,Pd₂ X₂ (μ-dppm)₂, wherein dppm=bis(diphenylphosphino)methane, wherein Xis selected from Cl, Br, or I, introducing H₂ S to the palladium complexin solution whereby sulfur is abstracted by the complex from the H₂ S toform Pd₂ X₂ (μ-S)(μ-dppm)₂ and hydrogen.
 2. The process according toclaim 1 comprising the additional step of oxidizing the Pd₂ X₂(μ-S)(μ-dppm)₂ to form a (μ-SO₂) complex of the formula Pd₂ X₂(μ-SO₂)(μ-dppm)₂, whereby the (μ-SO₂) complex spontaneously loses SO₂ toregenerate the palladium complex.
 3. The process according to claim 2comprising in addition the step of:introducing N₂ gas to the (μ-SO₂)complex to facilitate the loss of SO₂ to regenerate the palladiumcatalyst.
 4. A process for abstracting sulfur from an H₂ S containinggas and forming hydrogen comprising establishing a palladium complex ofthe formula ##STR3## wherein P P=bis(diphenylphosphino)methane, whereinX is selected from Cl, Br, or I,introducing H₂ S to the palladiumcomplex in solution, reacting the palladium complex with the H₂ Saccording to the reaction ##STR4## whereby sulfur is abstracted andhydrogen is formed.
 5. The process according to claim 4 comprising theadditional step of oxidizing the ##STR5## by exposure to an oxidant toform an SO₂ adduct, the SO₂ adduct tending to spontaneously lose SO₂ toregenerate the palladium complex.
 6. The process according to claim 5comprising in addition the step of regenerating the palladium complex byremoval means causing the SO₂ adduct to lose SO₂, the removal meansselected from introducing N₂ gas, applying heat, or applying reducedpressure.